This invention relates to a photodetector carrier and a photodetector module having such carrier.
On the receiving side of the common optical communication system, an optical signal transmitted by way of an optical transmission line (optical fiber) is photoelectrically converted to obtain an electric signal by means of a photodetector such as a photodiode, and information is reproduced with such electrical signal. A photodetector module which is used in such system includes a photodetector and an optical fiber arranged in a predetermined positional relationship. Since such photodetector is mounted onto a carrier made of a ceramic material or the like and then the carrier is fixed to a case of the photodetector module, a mounting structure of the photodetector carrier onto a case which has a superior aging characteristic under a high temperature circumstance is demanded in order to maintain an optical coupling characteristic between the optical fiber and the photodetector for a long period of time.
An extemplary one of conventional photodetector carriers is shown in FIG. 1. Referring to FIG. 1, a photodetector carrier 2 is made of a ceramic material, and a photodetector mounting portion 4 and wiring patterns 6 are formed on the photodetector carrier 2 by metallization. Further, a rear face 2a of the carrier 2 is processed by metallization over an entire area thereof in order to facilitate fixation of the carrier 2 to a case of a photodetector module.
Referring now to FIG. 2, there is shown a schematic view of the photodetector carried mounted on a case of a photodetector module in accordance with a known method. A photodiode (PD) chip 8 is die-bonded to the photodetector mounting portion 4 not shown in FIG. 2 of the carrier 2 by way of a soldering material such as AuSn having a melting point of 300.degree. C. or while the carried 2 is secured by soldering to a case 12 by way of another solder having a melting point lower than that of the soldering material such as, for example, Sn-Pb eutectic solder. Reference numeral 10 denotes an optical fiber, and signal light from the optical fiber 10 is photoelectrically converted into an electric signal by the PD chip 8, and the electric signal is transmitted by way of the wiring patterns 6 and bonding wires not shown to another wiring pattern on a substrate not shown on which various electric parts not shown are mounted. The reason why the solder having a low melting point is employed for the fixation of the carrier 2 is that, since the carrier 2 is fixed to the case 12 after the PD chip 8 is mounted onto the carrier 2, if such solder having a low melting point is not employed for the fixation of the carrier 2, there is the possibility that the PD chip 8 which has been fixed once to the carrier 2 may come out of position.
Since such solder having a low melting point which is used for the fixation of the carrier 2 commonly exhibits a considerable creep, it is disadvantageous in that the aging characteristic thereof is interior under a high temperature environment. Such inferiority in aging characteristic will result in deterioration in optical coupling characteristic between the optical fiber and the PD chip. Further, according to the structure of the conventional carrier, a ceramic material having a dielectric constant .epsilon. is held between a wiring pattern and the case, and consequently, the carrier has an electric capacitance. Accordingly, the carrier has a disadvantage that such electric capacitance thereof will deteriorate the high speed characteristics of the PD chip.